The present invention relates in general to semiconductor devices and, more particularly, to integrated circuits having a high voltage stage formed with low breakdown voltage transistors.
Many integrated circuits use physically small transistors to achieve a high functional density while maintaining a small die size and low cost. For example, some integrated circuits are formed with metal-oxide-semiconductor field effect transistors (MOSFETS) whose source and drain diffusions are formed as shallow, heavily doped junctions with small feature sizes. The source and drain diffusions are disposed in a well region of the opposite conductivity type and typically have low drain to well region avalanche breakdown voltages.
An integrated circuit formed with low drain to well region breakdown voltage transistors has a disadvantage of being limited to operating with low supply voltages in order to avoid damage to the integrated circuit. However, some applications require signals generated by the integrated circuit to operate at a voltage level greater than the drain to well region breakdown voltage of the individual transistors of the integrated circuit.
Some previous integrated circuits increase a transistor""s breakdown voltage by increasing the complexity of the fabrication process. For example, some processes add photomasking and/or diffusion steps to form a lightly doped drain region between the channel region and the heavily doped drain electrode in order to increase the breakdown voltage. However, the additional processing steps increase the cost of the integrated circuit. Moreover, these lightly doped drains increase the on resistance of the transistor, which degrades performance. Other schemes use circuit techniques to increase the breakdown voltage of a stage, but such schemes require a large die area, and therefore result in a high fabrication cost.
Hence, there is a need for a device and method which increases the operating voltage of a portion of an integrated circuit while maintaining a small die area and avoiding the need for additional processing steps in order to maintain a low manufacturing cost.